Role of 3-O-sulfated heparan sulfate in virus-induced polykaryocyte formation

Glycosaminoglycan microarrays for studying glycosaminoglycan-protein systems

More than 3000 proteins are now known to bind to glycosaminoglycans (GAGs). Yet, GAG-protein systems are rather poorly understood in terms of selectivity of recognition, molecular mechanism of action, and translational promise. High-throughput screening (HTS) technologies are critically needed for studying GAG biology and developing GAG-based therapeutics. Microarrays, developed within the past two decades, have now improved to the point of being the preferred tool in the HTS of biomolecules. GAG microarrays, in which GAG sequences are immobilized on slides, while similar to other microarrays, have their own sets of challenges and considerations. GAG microarrays are rapidly becoming the first choice in studying GAG-protein systems. Here, we review different modalities and applications of GAG microarrays presented to date. We discuss advantages and disadvantages of this technology, explain covalent and non-covalent immobilization strategies using different chemically reactive groups, and present various assay formats for qualitative and quantitative interpretations, including selectivity screening, binding affinity studies, competitive binding studies etc. We also highlight recent advances in implementing this technology, cataloging of data, and project its future promise. Overall, the technology of GAG microarray exhibits enormous potential of evolving into more than a mere screening tool for studying GAG - protein systems.

Archaic connectivity between the sulfated heparan sulfate and the herpesviruses - An evolutionary potential for cross-species interactions

The structural diversity of metazoic heparan sulfate (HS) composed of unique sulfated domains is remarkably preserved among various vertebrates and invertebrate species. Interestingly the sulfated moieties of HS have been known as the key determinants generating extraordinary ligand binding sites in the HS chain to regulate multiple biological functions and homeostasis. One such ligand for 3-O sulfation in the HS chain is a glycoprotein D (gD) from an ancient herpesvirus, herpes simplex virus (HSV). This interaction between gD and 3-O sulfated HS leads to virus-cell fusion to promote HSV entry. It is quite astonishing that HSV-1, which infects two-thirds of the world population, is also capable of causing severe diseases in primates and non-primates including primitive zebrafish. Supporting evidence that HSV may cross the species barrier comes from the fact that an enzymatic modification in HS encoded by 3-O sulfotransferase-3 (3-OST-3) from a vertebrate zoonotic species enhances HSV-1 infectivity. The latter phenomenon suggests the possible role of sulfated-HS as an entry receptor during reverse zoonosis, especially during an event when humans encounter domesticated animals in proximity. In this mini-review, we explore the possibility that structural diversity in HS may have played a substantial role in species-specific adaptability for herpesviruses in general including their potential role in promoting cross-species transmission.

Herpes Simplex Virus Cell Entry Mechanisms: An Update

Herpes simplex virus (HSV) can infect a broad host range and cause mild to life threating infections in humans. The surface glycoproteins of HSV are evolutionarily conserved and show an extraordinary ability to bind more than one receptor on the host cell surface. Following attachment, the virus fuses its lipid envelope with the host cell membrane and releases its nucleocapsid along with tegument proteins into the cytosol. With the help of tegument proteins and host cell factors, the nucleocapsid is then docked into the nuclear pore. The viral double stranded DNA is then released into the host cell’s nucleus. Released viral DNA either replicates rapidly (more commonly in non-neuronal cells) or stays latent inside the nucleus (in sensory neurons). The fusion of the viral envelope with host cell membrane is a key step. Blocking this step can prevent entry of HSV into the host cell and the subsequent interactions that ultimately lead to production of viral progeny and cell death or latency. In this review, we have discussed viral entry mechanisms including the pH-independent as well as pH-dependent endocytic entry, cell to cell spread of HSV and use of viral glycoproteins as an antiviral target.

Preferential recognition and antagonism of SARS-CoV-2 spike glycoprotein binding to 3-O-sulfated heparan sulfate

The COVID-19 pandemic caused by SARS-CoV-2 is in immediate need of an effective antidote. Although the Spike glycoprotein (SgP) of SARS-CoV-2 has been shown to bind to heparins, the structural features of this interaction, the role of a plausible heparan sulfate proteoglycan (HSPG) receptor, and the antagonism of this pathway through small molecules remain unaddressed. Using an in vitro cellular assay, we demonstrate HSPGs modified by the 3-O-sulfotransferase isoform-3, but not isoform-5, preferentially increased SgP-mediated cell-to-cell fusion in comparison to control, unmodified, wild-type HSPGs. Computational studies support preferential recognition of the receptor-binding domain of SgP by 3-O-sulfated HS sequences. Competition with either fondaparinux, a 3-O-sulfated HS-binding oligopeptide, or a synthetic, non-sugar small molecule, blocked SgP-mediated cell-to-cell fusion. Finally, the synthetic, sulfated molecule inhibited fusion of GFP-tagged pseudo SARS-CoV-2 with human 293T cells with sub-micromolar potency. Overall, overexpression of 3-O-sulfated HSPGs contribute to fusion of SARS-CoV-2, which could be effectively antagonized by a synthetic, small molecule.

A synthetic glycosaminoglycan mimetic blocks HSV-1 infection in human iris stromal cells

Herpes simplex virus type-1 (HSV-1) is a significant pathogen that affects vision by targeting multiple regions in the human eye including iris. Using a focused library of synthetic non-saccharide glycosaminoglycan mimetics (NSGMs), we identified sulfated pentagalloylglucoside (SPGG) as a potent inhibitor of HSV-1 entry and cell-to-cell spread in the primary cultures of human iris stromal (HIS) cells isolated from eye donors. Using in vitro β-galactosidase reporter assay and plaque reduction assay, SPGG was found to inhibit HSV-1 entry in a dosage-dependent manner (IC₅₀ ∼6.0 μM). Interestingly, a pronounced inhibition in HSV-1 entry and spread was observed in HIS cells, or a cell line expressing specific gD-receptor, when virions were pre-treated with mimetics suggesting a possible interaction between SPGG and the HSV-1 glycoprotein. To examine the significance of gD-SPGG interaction, HIS cells were pretreated with SPGG, which showed a significant reduction in gD binding. Taken together, our results provide strong evidence of SPGG being a novel viral entry inhibitor against ocular HSV infection.

Sulfotransferase and Heparanase: Remodeling Engines in Promoting Virus Infection and Disease Development

An extraordinary binding site generated in heparan sulfate (HS) structures, during its biosynthesis, provides a unique opportunity to interact with multiple protein ligands including viral proteins, and therefore adds tremendous value to this master molecule. An example of such a moiety is the sulfation at the C3 position of glucosamine residues in HS chain via 3-O sulfotransferase (3-OST) enzymes, which generates a unique virus-cell fusion receptor during herpes simplex virus (HSV) entry and spread. Emerging evidence now suggests that the unique patterns in HS sulfation assist multiple viruses in invading host cells at various steps of their life cycles. In addition, sulfated-HS structures are known to assist in invading host defense mechanisms and initiating multiple inflammatory processes; a critical event in the disease development. All these processes are detrimental for the host and therefore raise the question of how HS-sulfation is regulated. Epigenetic modulations have been shown to be implicated in these reactions during HSV infection as well as in HS modifying enzyme sulfotransferases, and therefore pose a critical component in answering it. Interestingly, heparanase (HPSE) activity is shown to be upregulated during virus infection and multiple other diseases assisting in virus replication to promote cell and tissue damage. These phenomena suggest that sulfotransferases and HPSE serve as key players in extracellular matrix remodeling and possibly generating unique signatures in a given disease. Therefore, identifying the epigenetic regulation of OST genes, and HPSE resulting in altered yet specific sulfation patterns in HS chain during virus infection, will be a significant a step toward developing potential diagnostic markers and designing novel therapies.

HSV-1 interaction to 3-O-sulfated heparan sulfate in mouse-derived DRG explant and profiles of inflammatory markers during virus infection

The molecular mechanism of herpes simplex virus (HSV) entry and the associated inflammatory response in the nervous system remain poorly understood. Using mouse-derived ex vivo dorsal root ganglia (DRG) explant model and single cell neurons (SCNs), in this study, we provided a visual evidence for the expression of heparan sulfate (HS) and 3-O-sulfated heparan sulfate (3-OS HS) followed by their interactions with HSV-1 glycoprotein B (gB) and glycoprotein D (gD) during cell entry. Upon heparanase treatment of DRG-derived SCN, a significant inhibition of HSV-1 entry was observed suggesting the involvement of HS role during viral entry. Finally, a cytokine array profile generated during HSV-1 infection in DRG explant indicated an enhanced expression of chemokines (LIX, TIMP-2, and M-CSF)—known regulators of HS. Taken together, these results highlight the significance of HS during HSV-1 entry in DRG explant. Further investigation is needed to understand which isoforms of 3-O-sulfotransferase (3-OST)-generated HS contributed during HSV-1 infection and associated cell damage.

Diversity of heparan sulfate and HSV entry: basic understanding and treatment strategies

A modified form of heparan sulfate (HS) known as 3-O-sulfated heparan sulfate (3-OS HS) generates fusion receptor for herpes simplex virus (HSV) entry and spread. Primary cultures of corneal fibroblasts derived from human eye donors have shown the clinical significance of this receptor during HSV corneal infection. 3-OS HS- is a product of a rare enzymatic modification at C3 position of glucosamine residue which is catalyzed by 3-O-sulfotransferases (3-OSTs) enzymes. From humans to zebrafish, the 3-OST enzymes are highly conserved and widely expressed in cells and tissues. There are multiple forms of 3-OSTs each producing unique subset of sulfated HS making it chemically diverse and heterogeneous. HSV infection of cells or zebrafish can be used as a unique tool to understand the structural-functional activities of HS and 3-OS HS and likewise, the infection can be used as a functional assay to screen phage display libraries for identifying HS and 3-OS HS binding peptides or small molecule inhibitors. Using this approach over 200 unique 12-mer HS and 3-OS HS recognizing peptides were isolated and characterized against HSV corneal infection where 3-OS HS is known to be a key receptor. In this review we discuss emerging role of 3-OS HS based therapeutic strategies in preventing viral infection and tissue damage.

Zebrafish: modeling for herpes simplex virus infections

For many years, zebrafish have been the prototypical model for studies in developmental biology. In recent years, zebrafish has emerged as a powerful model system to study infectious diseases, including viral infections. Experiments conducted with herpes simplex virus type-1 in adult zebrafish or in embryo models are encouraging as they establish proof of concept with viral-host tropism and possible screening of antiviral compounds. In addition, the presence of human homologs of viral entry receptors in zebrafish such as 3-O sulfated heparan sulfate, nectins, and tumor necrosis factor receptor superfamily member 14-like receptor bring strong rationale for virologists to test their in vivo significance in viral entry in a zebrafish model and compare the structure-function basis of virus zebrafish receptor interaction for viral entry. On the other end, a zebrafish model is already being used for studying inflammation and angiogenesis, with or without genetic manipulations, and therefore can be exploited to study viral infection-associated pathologies. The major advantage with zebrafish is low cost, easy breeding and maintenance, rapid lifecycle, and a transparent nature, which allows visualizing dissemination of fluorescently labeled virus infection in real time either at a localized region or the whole body. Further, the availability of multiple transgenic lines that express fluorescently tagged immune cells for in vivo imaging of virus infected animals is extremely attractive. In addition, a fully developed immune system and potential for receptor-specific knockouts further advocate the use of zebrafish as a new tool to study viral infections. In this review, we focus on expanding the potential of zebrafish model system in understanding human infectious diseases and future benefits.

Members of 3-O-Sulfotransferases (3-OST) Family: A Valuable Tool from Zebrafish to Humans for Understanding Herpes Simplex Virus Entry

The journey of many viruses to infect cells begins when the virus first binds to cell surface heparan sulfate (HS). The initial step of cell attachment or binding during herpes simplex virus type-1 (HSV-1) entry is mediated by envelope glycoprotein B (gB) and C (gC). The binding is followed by fusion between virus envelope and cell membrane during which HSV-1 glycoprotein D (gD) interacts with a modified form of HS know as 3-O-sulfated heparan sulfate (3-OS HS). The rare modification of 3-O-sulfation on HS chain is governed by enzymes known as 3-O-sulfotransferase (3-OST). Currently, there are seven isoforms of human 3-OSTs that have been identified, and with the exception of 3-OST-1, all other 3-OST isoforms allow HSV-1 entry and spread. Recently, the product of the zebrafish (ZF)-encoded 3-OST-3 was also recognized as a gD receptor, which mediates HSV-1 entry and cell-cell fusion similar to human 3-OST-3. Interestingly, the ZF system expresses multiple isoforms of 3-OST which could be very useful for studying the involvement of HS and 3-OS HS in virus tropism and virus-induced inflammation. In addition, therapeutic targeting of 3-OST generated HS is likely to bring about novel interventions against HSV-1. In this review we have taken a closer look at the potential of both human and ZF encoded 3-OSTs as valuable tools in HSV entry and inflammation studies.

A 3-O-sulfated heparan sulfate binding peptide preferentially targets herpes simplex virus 2-infected cells

Herpes simplex virus 2 (HSV-2) is the primary cause of genital herpes, which is one of the most common sexually transmitted viral infections worldwide and a major cofactor for human immunodeficiency virus infection. The lack of an effective vaccine or treatment and the emergence of drug-resistant strains highlight the need for developing new antivirals for HSV-2. Here, we demonstrate that a low-molecular-weight peptide isolated against 3-O-sulfated heparan sulfate (3-OS HS) can efficiently block HSV-2 infection. Treatment with the peptide inhibited viral entry and cell-to-cell spread both in vitro and in vivo using a mouse model of genital HSV-2 infection. Quite interestingly, the peptide showed a preferential binding to HSV-2-infected cells, with more than 200% increased binding compared to uninfected cells. Our additional results show that heparan sulfate expression is upregulated by 25% upon HSV-2 infection, which is a significant new finding that could be exploited for designing new diagnostic tests and treatment strategies against HSV-2-infected cells. In addition, our results also raise the possibility that 3-OS HS modifications within HS may be upregulated even more to accommodate for a significantly higher increase in the peptide binding to the infected cells.

Herpes simplex virus type-1 (HSV-1) entry into human mesenchymal stem cells is heavily dependent on heparan sulfate

Hematopoietic stem cells recipients remain susceptible to opportunistic viral infections including herpes simplex virus type-1 (HSV-1). The purpose of this investigation was to analyze susceptibility of human mesenchymal stem cells (hMSCs) to HSV-1 infection and identify the major entry receptor. Productive virus infection in hMSCs was confirmed by replication and plaque formation assays using a syncytial HSV-1 KOS (804) virus. To examine the significance of entry receptors, RT-PCR and antibody-blocking assays were performed. RT-PCR data showed the expression of gD receptors: nectin-1, 3-O sulfotransferase-3 (3-OST-3), and HVEM. Antibody-blocking assay together with heparinase treatment suggested an important role for HS and 3-O-sulfated heparan sulfate (3-OS HS), but not nectin-1 or HVEM, in mediating HSV-1 entry and spread in hMSCs. Taken together, our results provide strong evidence demonstrating that HSV-1 is capable of infecting hMSCs and HS and 3-OS HS serve as its entry receptors during this process.

Anti-heparan sulfate peptides that block herpes simplex virus infection in vivo

Heparan sulfate (HS) and its highly modified form, 3-O-sulfated heparan sulfate (3-OS HS), contribute strongly to herpes simplex virus type-1 (HSV-1) infection in vitro. Here we report results from a random M13-phage display library screening to isolate 12-mer peptides that bind specifically to HS, 3-OS HS, and block HSV-1 entry. The screening identified representative candidates from two-different groups of anti-HS peptides with high positive charge densities. Group 1, represented by G1 peptide (LRSRTKIIRIRH), belongs to a class with alternating charges (XRXRXKXXRXRX), and group 2, represented by G2 peptide (MPRRRRIRRRQK), shows repetitive charges (XXRRRRXRRRXK). Viral entry and glycoprotein D binding assays together with fluorescent microscopy data indicated that both G1 and G2 were potent in blocking HSV-1 entry into primary cultures of human corneal fibroblasts and CHO-K1 cells transiently expressing different glycoprotein D receptors. Interestingly, G2 peptide isolated against 3-OS HS displayed wider ability to inhibit entry of clinically relevant strains of HSV-1 and some divergent members of herpesvirus family including cytomegalovirus and human herpesvirus-8. To identify functional residues within G1 and G2, we performed point mutations and alanine-scanning mutagenesis. Several arginine and a lysine residues were needed for anti-HSV-1 activity, suggesting the importance of the positively charged residues in virus-cell binding and virus-induced membrane fusion. In vivo administration of G1 or G2 peptide as a prophylactic eye drop completely blocked HSV-1 spread in the mouse cornea as evident by immunohistochemistry. This result also highlights an in vivo significance of HS and 3-OS HS during ocular herpes infection.

In vitro evaluation of antiviral and virucidal activity of a high molecular weight hyaluronic acid

Background

hyaluronic acid (HA), a non-sulphated glycosaminoglycan, is present in synovial fluid, vitreous humour serum and many connective tissues. Pharmaceutical preparations of HA are used in clinical practice for wound healing, joint pain, kerato-conjunctivitis, asthma, mouth care, oesophageal-reflux, and gastritis. Moreover, it is used as a filler to counteract ageing and facial lipoatrophy. Our study aims at investigating the in vitro antiviral activity of a high molecular weight HA.

Methods

the MTT test was used to rule out the potential toxic effects of HA on the different cell lines used in the antiviral assays. The antiviral activity of HA against Coxsackievirus B5, Herpes Simplex Virus-1, Mumps Virus, Adenovirus-5, Influenza Virus A/H1N1, Human Herpesvirus-6, Porcine Parvovirus, Porcine Reproductive and Respiratory Syndrome Virus was assessed by virus yield assays.

Results

the most effective inhibition was observed against Coxsackievirus B5, with 3Log reduction of the virus yield at 4 mg/ml, and a reduction of 3.5Log and 2Log, at 2 mg/ml and 1 mg/ml, respectively: the selectivity index was 16. Mumps virus was highly inhibited too showing a reduction of 1.7Log at 1 mg/ml and 1Log at 4 mg/ml and 2 mg/ml (selectivity index = 12). The selectivity index for Influenza Virus was 12 with the highest inhibition (1Log) observed at 4 mg/ml. Herpes Simplex Virus-1 and Porcine Parvovirus were mildly inhibited, whereas no antiviral activity was observed with respect to Adenovirus-5, Human Herpesvirus-6, Porcine Reproductive and Respiratory Syndrome Virus. No HA virucidal activity was ever observed against any of the viruses tested. Kinetic experiments showed that both Coxsackievirus B5 and Herpes simplex virus-1 replication were consistently inhibited, not influenced by the time of HA addition, during the virus replication cycle.

Conclusions

the spectrum of the antiviral activity exhibited by HA against both RNA and DNA viruses, known to have different structures (with or without envelope) and replication strategies, suggests a non specific mechanism of action, probably involving cell membrane-virus interaction steps. The results of the kinetic experiments support this hypothesis.

Phosphoinositide 3 kinase signalling may affect multiple steps during herpes simplex virus type-1 entry

Early interactions of herpes simplex virus type-1 (HSV-1) with cells lead to cytoskeletal changes facilitating filopodia formation and membrane fusion. Here, we demonstrate that phosphoinositide 3 kinase (PI3K) signalling may affect multiple steps during HSV-1 entry. An inhibitor of PI3K (LY294002) blocked HSV-1 entry and the blockage was cell-type- and gD receptor-independent. Entry inhibition was also observed with primary cultures of the human corneal fibroblasts and unrelated β- and γ-herpesviruses. Immunofluorescence analysis demonstrated that LY294002 negatively affected HSV-1-induced filopodia formation. Similar effects of the inhibitor were seen on HSV-1 glycoprotein-induced cell-to-cell fusion. Cells expressing HSV-1 glycoproteins (gB, gD, gH and gL) showed significantly less fusion with target cells in the presence of the inhibitor. Expression of a dominant-negative PI3K mutant negatively affected both entry and fusion. We also show that inhibition of PI3K signalling also affected RhoA activation required for HSV-1 entry into certain cell types.

Zebrafish-encoded 3-O-sulfotransferase-3 isoform mediates herpes simplex virus type 1 entry and spread

Heparan sulfate proteoglycans modified by human glucosaminyl 3-O-sulfotransferase-3 (3-OST-3) isoform generates the cellular receptor for herpes simplex virus type 1 (HSV-1). Interestingly, the ability of zebrafish (ZF)-encoded 3-OST-3 isoform to modify heparan sulfate to mediate HSV-1 entry and cell-cell fusion has not been determined although it is predominantly expressed in ZF, a popular model organism to study viral infections. Here, we demonstrate that expression of ZF-encoded 3-OST-3 isoform renders the resistant Chinese hamster ovary (CHO-K1) cells to become susceptible for HSV-1 entry. The following lines of evidence support the important role of ZF-encoded 3-OST-3 isoform as the mediator of HSV-1 entry into CHO-K1 cells: (1) ZF 3-OST-3-expressing CHO-K1 cells were able to preferentially bind HSV-1 glycoprotein D, and (2) CHO-K1 cells expressing ZF-encoded 3-OST-3 acquire the ability to fuse with cells expressing HSV-1 glycoproteins. Finally, knocking down 3-OST-3 receptor by siRNA in ZF fibroblasts cells significantly reduced HSV-1 entry and glycoprotein D binding to cells. Taken together, our results provide novel insight into the significance of ZF 3-OST-3 isoform as an HSV-1 entry and fusion receptor and its potential involvement in the HSV-1 disease model of ZF.

In vitro antiviral activity of neem (Azardirachta indica L.) bark extract against herpes simplex virus type-1 infection

Herpes simplex virus type 1 (HSV-1) causes significant health problems from periodical skin and corneal lesions to encephalitis. We report here that an aqueous extract preparation from the barks of neem plant Azardirachta indica acts as a potent entry inhibitor against HSV-1 infection into natural target cells. The neem bark extract (NBE) significantly blocked HSV-1 entry into cells at concentrations ranging from 50 to 100 microg/ml. The blocking activity of NBE was observed when the extract was pre-incubated with the virus but not with the target cells, suggesting a direct antiHSV-1 property of the neem bark. Further, virions treated with NBE failed to bind the cells which implicate a role of NBE as an attachment step blocker. Cells treated with NBE also inhibited HSV-1 glycoprotein-mediated cell-cell fusion and polykaryocytes formation suggesting an additional role of NBE at the viral fusion step. These findings open a potential new avenue for the development of NBE as a novel antiherpetic microbicide.

Viral entry mechanisms: cellular and viral mediators of herpes simplex virus entry

Herpes simplex virus type-1 and type-2 are highly prevalent human pathogens causing life-long infections. The process of infection begins when the virions bind heparan sulfate moieties present on host cell surfaces. This initial attachment then triggers a cascade of molecular interactions involving multiple viral and host cell proteins and receptors, leading to penetration of the viral nucleocapsid and tegument proteins into the cytoplasm. The nucleocapsid is then transported to the nuclear membrane and the viral DNA is released for replication in the nucleus. Recent studies have revealed that herpes simplex virus entry or penetration into cells may be a highly complex process and the mechanism of entry may demonstrate unique cell-type specificities. Although specificities clearly exist, past and ongoing studies demonstrate that herpes simplex virus may share certain common receptors and pathways that are also used by many other human viruses. This minireview helps to shed light on recent revelations on the herpes simplex virus entry process.

Synthesis of heparan sulfate with cyclophilin B-binding properties is determined by cell type-specific expression of sulfotransferases

Cyclophilin B (CyPB) induces migration and adhesion of T lymphocytes via a mechanism that requires interaction with 3-O-sulfated heparan sulfate (HS). HS biosynthesis is a complex process with many sulfotransferases involved. N-Deacetylases/N-sulfotransferases are responsible for N-sulfation, which is essential for subsequent modification steps, whereas 3-O-sulfotransferases (3-OSTs) catalyze the least abundant modification. These enzymes are represented by several isoforms, which differ in term of distribution pattern, suggesting their involvement in making tissue-specific HS. To elucidate how the specificity of CyPB binding is determined, we explored the relationships between the expression of these sulfotransferases and the generation of HS motifs with CyPB-binding properties. We demonstrated that high N-sulfate density and the presence of 2-O- and 3-O-sulfates determine binding of CyPB, as evidenced by competitive experiments with heparin derivatives, soluble HS, and anti-HS antibodies. We then showed that target cells, i.e. CD4+ lymphocyte subsets, monocytes/macrophages, and related cell lines, specifically expressed high levels of NDST2 and 3-OST3 isoforms. Silencing the expression of NDST1, NDST2, 2-OST, and 3-OST3 by RNA interference efficiently decreased binding and activity of CyPB, thus confirming their involvement in the biosynthesis of binding sequences for CyPB. Moreover, we demonstrated that NDST1 was able to partially sulfate exogenous substrate in the absence of NDST2 but not vice versa, suggesting that both isoenzymes do not have redundant activities but do have rather complementary activities in making N-sulfated sequences with CyPB-binding properties. Altogether, these results suggest a regulatory mechanism in which cell type-specific expression of certain HS sulfotransferases determines the specific binding of CyPB to target cells.

An unusual dependence of human herpesvirus-8 glycoproteins-induced cell-to-cell fusion on heparan sulfate

Human herpesvirus-8 (HHV-8) is known to interact with cell surface heparan sulfate (HS) for entry into a target cell. Here we investigated the role of HS during HHV-8 glycoproteins-induced cell fusion. Interestingly, the observed fusion demonstrated an unusual dependence on HS as evident from following lines of evidence: (1) a significant reduction in cell-to-cell fusion occurred when target cells were treated with heparinase; (2) in a competition assay, when the effector cells expressing HHV-8 glycoproteins were challenged with soluble HS, cell-to-cell fusion was reduced; and, (3) co-expression of HHV-8 glycoproteins gH-gL on target cells resulted in inhibition of cell surface HS expression. Taken together, our results indicate that cell surface HS can play an additional role during HHV-8 pathogenesis.

A sugar binding protein cyanovirin-N blocks herpes simplex virus type-1 entry and cell fusion

Herpes simplex virus type-1 (HSV-1) causes significant health problems from periodic skin and corneal lesions to encephalitis. It is also considered a cofactor in the development of age-related secondary glaucoma. Inhibition of HSV-1 at the stage of viral entry generates a unique opportunity for preventative and/or therapeutic intervention. Here we provide evidence that a sugar binding antiviral protein, cyanovirin-N (CV-N), can act as a potent inhibitor of HSV-1 entry into natural target cells. Inhibition of entry was independent of HSV-1 gD receptor usage and it was observed in transformed as well as primary cell cultures. Evidence presented herein suggests that CV-N can not only block virus entry to cells but also, it is capable of significantly inhibiting membrane fusion mediated by HSV glycoproteins. While CV-N treated virions were significantly deficient in entering into cells, HSV-1 glycoproteins-expressing cells pretreated with CV-N demonstrated reduced cell-to-cell fusion and polykaryocytes formation. The observation that CV-N can block both entry as well as membrane fusion suggests a stronger potential for this compound in antiviral therapy against HSV-1.

The Importance of Heparan Sulfate in Herpesvirus Infection

Herpes simplex virus type-1 (HSV-1) is one of many pathogens that use the cell surface glycosaminoglycan heparan sulfate as a receptor. Heparan sulfate is highly expressed on the surface and extracellular matrix of virtually all cell types making it an ideal receptor. Heparan sulfate interacts with HSV-1 envelope glycoproteins gB and gC during the initial attachment step during HSV-1 entry. In addition, a modified form of heparan sulfate, known as 3-O-sulfated heparan sulfate, interacts with HSV-1 gD to induce fusion between the viral envelope and host cell membrane. The 3-O-sulfation of heparan sulfate is a rare modification which occurs during the biosynthesis of heparan sulfate that is carried out by a family of enzymes known as 3-O-sulfotransferases. Due to its involvement in multiple steps of the infection process, heparan sulfate has been a prime target for the development of agents to inhibit HSV entry. Understanding how heparan sulfate functions during HSV-1 infection may not only be critical for inhibiting infection by this virus, but it may also be crucial in the fight against many other pathogens as well.

Role for nectin-1 in herpes simplex virus 1 entry and spread in human retinal pigment epithelial cells

Herpes simplex virus 1 (HSV-1) demonstrates a unique ability to infect a variety of host cell types. Retinal pigment epithelial (RPE) cells form the outermost layer of the retina and provide a potential target for viral invasion and permanent vision impairment. Here we examine the initial cellular and molecular mechanisms that facilitate HSV-1 invasion of human RPE cells. High-resolution confocal microscopy demonstrated initial interaction of green fluorescent protein (GFP)-tagged virions with filopodia-like structures present on cell surfaces. Unidirectional movement of the virions on filopodia to the cell body was detected by live cell imaging of RPE cells, which demonstrated susceptibility to pH-dependent HSV-1 entry and replication. Use of RT-PCR indicated expression of nectin-1, herpes virus entry mediator (HVEM) and 3-O-sulfotransferase-3 (as a surrogate marker for 3-O-sulfated heparan sulfate). HVEM and nectin-1 expression was subsequently verified by flow cytometry. Nectin-1 expression in murine retinal tissue was also demonstrated by immunohistochemistry. Antibodies against nectin-1, but not HVEM, were able to block HSV-1 infection. Similar blocking effects were seen with a small interfering RNA construct specifically directed against nectin-1, which also blocked RPE cell fusion with HSV-1 glycoprotein-expressing Chinese hamster ovary (CHO-K1) cells. Anti-nectin-1 antibodies and F-actin depolymerizers were also successful in blocking the cytoskeletal changes that occur upon HSV-1 entry into cells. Our findings shed new light on the cellular and molecular mechanisms that help the virus to enter the cells of the inner eye.

HVEM and nectin-1 are the major mediators of herpes simplex virus 1 (HSV-1) entry into human conjunctival epithelium

PURPOSE

The human conjunctiva is a natural target for herpes simplex virus (HSV)-1 infection. The goals of this study were to investigate the cellular and molecular mechanisms of HSV-1 entry into human conjunctival epithelial (HCjE) cells. Specific features of entry studied included the method of initial viral binding to cells, pH dependency, and expression and usage of specific HSV-1 entry receptors.

METHODS

To observe HSV-1 initial binding, live cell imaging was performed on HSV-1-infected HCjE cells. Reporter HSV-1 virions expressing beta-galactosidase were used to determine entry of wild-type HSV-1(KOS) and a mutant, HSV-1(KOS)Rid1, into HCjE cells. HSV-1 replication in HCjE cells was determined by plaque assays. Lysosomotropic agents were used to determine whether viral entry was pH dependent. Reverse transcription (RT)-PCR, flow cytometry, and immunohistochemistry were used to determine the expression of receptors. Receptor-specific siRNAs were used to define the role of individual entry receptors.

RESULTS

HSV-1 virions attach to filopodia present on HCjE cells and use them to reach the cell body for entry. Cultured HCjE cells demonstrate susceptibility to HSV-1 entry and form plaques confirming viral replication. Blocking vesicular acidification significantly reduces entry, implicating a pH-dependent mode of entry. Multiple assays confirm the expression of entry receptors nectin-1, HVEM, and 3-O-sulfated heparan sulfate (3-OS HS) on the HCjE cell membrane. Knocking down of gD receptors by siRNAs interference implicates nectin-1 and HVEM as the major mediators of entry.

CONCLUSIONS

HSV-1 entry into HCjE cells is a pH-dependent process that is aided by targeted virus travel on filopodia. HCjE cells express all three major entry receptors, with nectin-1 and HVEM playing the predominant role in mediating entry.